1. Field of the Invention
This invention relates generally to a communication apparatus, and more particularly to a digital communication apparatus that provides digital communication between a plurality of terminal units and a central site using standard cable television coaxial/fiber optic networks and plant facilities.
2. Background Information
Systems for providing digital communications from multiple user terminals to a central site are well known. Previous system implementations, which share common communication channels, suffer substantial reliability degradation in two main areas.
First, when one of the multiple terminals fails such that the terminal continuously transmits, communication from all the other terminals using that communication channel are effectively blocked. This type of system failure is known in the trade as xe2x80x9ccarrier lock-onxe2x80x9d.
Secondly, systems which share a communication channel between a large number of customer terminals and a central site are subject to interference from radio broadcast stations and other radio frequency interference. Radio broadcast stations, and certain other interferers, transmit their signals at high power levels and over large geographic areas. Those broadcast signals may be picked up by customer terminals and the attached cable network. The interference pickup from many customer terminals is carried by the common communication channel to the central site. The interfering signals may then sum to a large interfering signal which can degrade or block the communication channel. This type of system interference or blockage is known in the trade as xe2x80x9cinterference ingressxe2x80x9d or simply as xe2x80x9cingressxe2x80x9d.
Systems incorporating prior art have not been commercially viable because of poor reliability due to xe2x80x9ccarrier lock-onxe2x80x9d and xe2x80x9cingressxe2x80x9d.
The present invention provides digital communication between a large number of user terminals and a central site, over a standard cable television coaxial or fiber optic network, without the poor reliability of prior art systems. The advantages of such a system are apparent. Cable television companies want to use their existing networks to provide customers with additional services which require reliable digital communication from the customer""s site to a central location over one or more frequency channels.
The present invention provides two-way communication of digital information between a central site and a plurality of user terminals over standard coaxial or fiber optic cables. The central site receives a user address from a digital I/O control and transmits a conventional frequency shift keyed (FSK) signal to all of the user terminals. The user terminal with the correct preassigned address will respond to the signal from the central site and perform its programmed function. The user terminal then transmits an acknowledgment, or a digital message, back to the central site.
The return transmission is a dual FSK signal with suppressed carrier. The full message content is contained in each of the transmitted sidebands. The sidebands are separated sufficiently to prevent a narrow-band interfering signal from degrading or blocking both sidebands simultaneously.
The central site has a receiver with two frequency channels matching the subcarrier""s frequencies. The central site receiving apparatus includes a decoder that does not pass information received in the two frequency channels unless it is received at a certain level for a prescribed duration.
A continuous interfering carrier on either sideband and/or momentary interference on either or both sidebands cannot degrade or block data communications on the return signal. Therefore, carrier lock-on and ingress interference do not disrupt digital communication in the present invention.
Accordingly, it is an objective of the present invention to provide a two-way digital communication apparatus which permits reliable communication between many customer terminals and a central site on a common frequency channel over a standard coaxial or fiber optic cable.
A further objective of the present invention is to provide digital communication apparatus which prevents system degradation or blockage due to xe2x80x9ccarrier lock-onxe2x80x9d.
A still further objective of the present invention is to provide digital communication apparatus which prevents system degradation or blockage due to narrow-band interference ingress from radio broadcast stations and certain other narrow band interferers.
Yet another objective of the present invention is to provide digital communication apparatus which prevents system degradation or blockage due to wide-band impulse interference ingress.
An even further objective of the present invention is to provide digital communication apparatus which is immune to carrier lock-on and ingress, and is simple and economical in construction.
In accordance with these and other objects which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.